Aerobic metabolismOxidative metabolism

Aerobic metabolism produces ATP using oxygen, breaking down carbohydrate and fat completely through the Krebs cycle and the electron transport chain. It yields far more energy per unit of fuel than anaerobic pathways and produces only carbon dioxide and water as waste.

It matters because it powers everything from resting metabolism to any effort lasting more than a couple of minutes, and because fat can only be burned aerobically. It is also what handles recovery between hard efforts, which is why aerobic fitness improves repeated-sprint performance in sports that look entirely anaerobic.

In practice its limitation is speed: it cannot supply ATP fast enough for maximal efforts, which is why the anaerobic systems exist at all. The two always run together, with the mix shifting by intensity rather than switching over.